In conventional audio systems, a channel-based approach is employed. Each channel may for example represent the content of one speaker or one speaker array. Possible coding schemes for such systems include discrete multi-channel coding or parametric coding such as MPEG Surround.
More recently, a new approach has been developed. This approach is object-based. In system employing the object-based approach, a three-dimensional audio scene is represented by audio objects with their associated positional metadata. These audio objects move around in the three-dimensional audio scene during playback of the audio signal. The system may further include so called bed channels, which may be described as stationary audio objects which are directly mapped to the speaker positions of for example a conventional audio system as described above.
A problem that may arise in an object-based audio system is how to efficiently encode and decode the audio signal and preserve the quality of the coded signal. A possible coding scheme includes, on an encoder side, creating a downmix signal comprising a number of channels from the audio objects and bed channels, and side information which enables recreation of the audio objects and bed channels on a decoder side.
MPEG Spatial Audio Object Coding (MPEG SAOC) describes a system for parametric coding of audio objects. The system sends side information, c.f. upmix matrix, describing the properties of the objects by means of parameters such as level difference and cross correlation of the objects. These parameters are then used to control the recreation of the audio objects on a decoder side. This process can be mathematically complex and often has to rely on assumptions about properties of the audio objects that is not explicitly described by the parameters. The method presented in MPEG SAOC may lower the required bitrate for an object-based audio system, but further improvements may be needed to further increase the efficiency and quality as described above.
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